1. Field of the Invention
The invention relates to a control apparatus for a hydraulic power steering system.
2. Description of the Related Art
Conventionally, a hydraulic power steering system that applies assist force to a steering system with the use of a hydraulic actuator such as a hydraulic cylinder has been widely known. For example, Japanese Patent Application Publication No. 2008-238882 (JP 2008-238882 A) describes a hydraulic power steering system in which an electric pump that is driven by a motor to generate hydraulic pressure is used as a hydraulic power source for a hydraulic actuator. In such a hydraulic power steering system, even when a steering operation is not performed, the electric pump usually keeps driving at a relatively low rotational speed (standby rotational speed). This makes it possible to apply assist force promptly, thereby improving the responsiveness.
When the electric pump is started from a standstill (a supply of electric power for driving the electric pump is started), for example, when an ignition switch (IG) is turned on, a large current instantaneously flows to the motor because inductive voltage has not been generated in the motor. Especially when the ambient temperature is low, the current that flows to the motor at the time of starting the electric pump is likely to be large because the viscosity of hydraulic fluid becomes high and therefore the load on the motor becomes high. Therefore, an in-vehicle power supply mounted in a vehicle is required to have a capacity high enough not to affect the operation of electric equipment even if a large amount of current is consumed by the electric pump at the time of, for example, turning on the ignition switch.
In recent years, for the purpose of further improving the responsiveness, providing a plurality of electric pumps in a hydraulic power steering system has been proposed. However, in this case, a significantly large amount of current is consumed at the time of, for example, turning on an ignition switch. Therefore, it is necessary to increase the capacity of an in-vehicle power supply depending on the number of electric pumps. As a result, for example, the cost of manufacturing the hydraulic power steering system may significantly increase.